Recently, various optometric devices and optical sensors for detecting the characteristics of substances using light have been proposed and also put into practical use. Examples of such optometric devices and optical sensors include a spectroscopic analysis apparatus for analyzing absorption spectra, a surface plasmon resonance (hereinafter, referred to as “SPR”) sensor, an optical sensor using a nonlinear emission phenomenon, and an optical sensor using a waveguide. These will be described in the following.
First, the spectroscopic analysis apparatus for absorption spectra will be described, which is used as a common spectrophotometer. The principle of this apparatus is that a spectrum generated by causing a monochromatic light beam to be transmitted through a given gas sample, liquid sample, or transparent solid sample is measured, and this spectrum is compared with the spectrum of a reference cell to detect an absorption peak unique to such a sample.
Next, the SPR sensor will be described. Recently, extensive research and development have been carried out on SPR sensors. SPR sensors utilize a surface plasmon excited at the interface between a metal film and a test substance to detect the characteristics of the test substance. SPR sensors have the characteristics that alteration of the test substance due to light will not occur, since the interaction between light and the test substance (for example, an aqueous solution) is limited to the vicinity of the surface of the metal film.
Next, the optical sensor using a nonlinear emission phenomenon will be described. When a substance is irradiated with incident light, light emission (nonlinear emission) having a frequency different from that of the irradiation light sometimes may be generated due to a nonlinear effect. This optical sensor uses this nonlinear emission to analyze a test substance. Analyses using a nonlinear emission phenomenon include fluorescence analysis, Raman scattering analysis, analysis using two-photon absorption fluorescence reaction, and analyses using second harmonic generation (hereinafter, referred to as “SHG”) and third harmonic generation (hereinafter, referred to as “THG”). The intensities of Raman scattering, two-photon absorption fluorescence reaction, SHG and THG, and so on are proportional to the square or cube of the electric field intensity. Accordingly, in order to efficiently cause the effects of these types of emission, it is necessary to increase the energy density of the incident light. Therefore, a method is also adopted in which nonlinear emission is caused by forming a focal point of incident light in a test substance using a lens, and the thus generated nonlinear emission is condensed for analysis.
Next, the optical sensor using a waveguide will be described. When a focal point of incident light is formed in a test substance using a lens, the region having a high light energy density is limited to near the focal point. Therefore, by producing a waveguide including a test substance as a core, and propagating light through that waveguide, it is possible to increase the energy density of light in the test substance over a long distance. By analyzing the light propagating through this waveguide, for example, it is possible to detect the characteristics of the test substance. In this case, the energy density of light is high, so that it is possible to perform highly accurate measurement. Furthermore, the measurement accuracy of the optical sensor using a waveguide is improved by increasing the length of the waveguide. The reason is that this allows an increase of the volume of a portion of the test substance that interacts with light. Thus, the measurement accuracy of the optical sensor using a waveguide can be improved easily. However, it is difficult to use air and an aqueous solution as the core of the waveguide, since they have a lower refractive index than a glass material, which is a typical component of a waveguide. Therefore, the optical sensor using a waveguide has a problem in that it is difficult to use air or an aqueous solution as the test substance.
Examples of the optical sensor using a nonlinear luminous phenomenon and the optical sensor using a waveguide are described in Non-patent Document 1 and Non-patent Document 2, for example.
The optical sensor described in Non-patent Document 1 includes a pair of adjacent slab waveguides. This optical sensor is configured such that the gap between the pair of slab waveguides is filled with a liquid test substance, light is propagated through the pair of slab waveguides to generate Raman scattering light, and this light is utilized to detect the characteristics of the test substance. The gap portion between the pair of slab waveguides that is filled with the test substance is an area in which the intensity of the propagation light becomes the highest, so that it is possible to perform highly accurate measurement. Furthermore, increasing the length of the slab waveguides can expand the region in which the interaction between light and the test substance occurs, making it possible to increase the intensity of Raman scattering light even further.
The optical sensor described in Non-patent Document 2 includes a hollow waveguide surrounded by a dielectric multilayer film. In this optical sensor, the hollow part of the hollow waveguide is filled with a liquid test substance to form a waveguide including the test substance as the core. Propagation light is confined within the core by the Bragg reflection effect of the surrounding multilayer film, and this propagation light is used to measure the test substance. Furthermore, it is also possible to confine fluorescence generated by supplying excitation light to the core portion within the core, and collect this from the end face of the waveguide. It is also possible to use this fluorescence to detect the characteristics of the test substance.
Non-patent Document 1: G Stanev, N Goutev, Zh S Nickolov, “Coupled waveguides for Raman studies of thin liquid films”, Appl. Phys. 31 (1998), p. 1782-1786
Non-patent Document 2: Dongliang Yin, David W. Deamer, Holger Schmidt, John P. Barber, Aaron R. Hawkins, “Integrated biophotonic sensor with single-molecule resolution”, CLEO/IQEC/PhAST, 2004, CThI4